This invention concerns injection molding nozzles used to inject plastic material into the cavity of a mold. Such nozzles receive molten plastic material from an injection molding machine and direct the same into a mold cavity through a passage called a gate. Two methods exist for this transfer: thermal, or open, gating; and valve gating.
In thermal gating, the gate is an open aperture through which plastic can pass during injection of plastic material. The gate is rapidly cooled at the end of the injection cycle to "freeze" the plastic material which remains in the gate to act as a plug to prevent drool of plastic material into the mold cavity when the mold is open for ejection of parts. In the next injection cycle, the cooling to the gate is removed and hot plastic material pushes the plug into the mold cavity, where it melts and mixes with the new melt stream.
In valve gating, gate opening and closing is independent of injection pressure and/or cooling, and is achieved mechanically, with a pin that travels back and forth, to open and close the gate.
Generally, valve gating is preferable to thermal gating because the gate mark left by valve gating on the finished molded part after injection is complete is much smaller than that which results from thermal gating. Larger gate sized can also be used in valve gate systems, leading to faster filling of the mold cavities and therefore shorter molding cycle times.
However, some disadvantages are frequently associate with valve gates. These disadvantages include "weld lines", which are areas where multiple melt flow fronts meet, and valve stem wear. Weld lines tend to introduce weakness or loss of mechanical strength into the finished part and result from the fact that the valve stem is surrounded by the plastic material, splitting the melt stream, which is later rejoined at the end of the stem, and this re-combining of the stream leads to weld lines. Hence, there exists a need for a gate design which allows for the melt stream, or streams in the case of two or more plastic materials, to remain separate while still being controlled with a common valve stem.
The valve stem is also subject to wear from mechanical stress, due to stem deflection from the incoming pressurized melt, and thermal stress, from constant contact with the melt. This wear is exacerbated in cases where reinforced plastic materials, i.e., those containing glass or other fibers or materials, are injected. Hence, there exists a need for a design which mitigates the wear of the valve stem.
The injection of two or more separate melt streams into a mold cavity, whether simultaneously or sequentially, is referred to as co-injection, and leads to layered wall structures in hollow articles and blow molding preforms. The prior art includes a multitude of processes and apparatuses for forming molded articles from multiple plastic materials by co-injection. For example, U.S. Pat Nos. 5,028,226 and 4,717,324 show simultaneous and sequential co-injection apparatuses and methods, respectively. Both patents show one nozzle dedicated to each mold cavity wherein the cavity is filled by injecting two or more resins through a single gate.
In the systems shown in each patent, a valve stem is used to prevent resin flow through the gate after injection is complete. In these systems, the hot runner systems employed to receive the various resins from their source for conveyance to the mold cavities are very complicated. Consequently, such hot runner systems lead to mold designs which are not compact and thereby allow fewer cavities and fewer articles to be molded within a given space on a molding machine.
U.K. Patent No. 1,369,744 discloses a sequential co-injection system using separate channels, commonly referred to as sprue channels, for each melt stream, and sliding shuttles which function as valve stems to open and close the connection between the injection machine and the channels. However, these separate melt channels converge into a single common gate area prior to injection, so that some potential for contamination between streams exists. Furthermore, the shuttles are hydraulically actuated, increasing the complexity of the nozzle and allowing the risk of leaking hydraulic fluid to contaminate the streams.
U.S. Pat. No. 4,470,936 also discloses a sequential co-injection system using separate sprue channels for each melt stream, with each sprue channel being independently heated and converging to a common gate. In this system, a shuttle ball or swing gate switches the flow of material from one sprue channel to the other. This system also suffers from the potential for contamination between streams, such as described above for U.K Patent 1,369,744. This is a special concern as wear of the shuttle ball or swing gate is likely in normal use.
U.S. Pat. No. 5,651,998, assigned to the assignee of the present invention, discloses a method and apparatus for either sequential or simultaneous co-injection utilizing two opposing injection nozzles on the core and cavity sides respectively of the mold. Although effective, this arrangement requires an additional injection nozzle which must also receive resin from an injection unit on the opposite (movable) mold core half. This arrangement significantly increases the space requirements for the mold and may not be acceptable in some applications.
U.S. Pat. No. 5,125,816 is similar to U.S. Pat. No. 5,651,998 in that sequential co-injection is achieved by opposing gates on both the mold core and cavity respectively. However, in this arrangement the moveable mold half is fitted with slide cores containing tubular passages for feeding resin to one half of the molded part These slide cores move via hydraulic cylinders to define secondary mold cavities, which are in turn filled by gates on the opposing mold half This system suffers from disadvantages due to its complexity, the additional mold hardware requirements, including the aforementioned slide cores and additional injection nozzles, and the need for special manufacturing attention due to tight tolerances.
U.S. Pat. No. 3,873,656 shows a co-injection apparatus wherein at least two plastics are injected into a mold cavity through different gates, using a valve gating system. This design is only suitable for molding very large plastic articles. Also, the hot runner system taught does not have the capability for allowing separate temperature control of the different resin types, which inherently limits the variety of resins that can be used together in one system Furthermore, since the gates are far apart from one another, the flow of each resin will not be symmetrical throughout the part, but instead will be biased in the area of the gate.
U.S. Pat. No. 4,289,191 shows injection molding of molten wax into a precision metal die, wherein hollow parts are molded to extremely tight tolerances of .+-.0.012 mm. The wax stream flows from a nozzle having a central bore to a cavity or space formed between the nozzle tip, which has a relief channel, and the socket on the exterior of the die, and then into two or more separate sprue ports that feed into the mold cavity. Control of wax flow is accomplished by a retractable plunger in the nozzle which functions like a conventional valve stem. Although more than one sprue port is employed to supply material to the mold, these ports are downstream of the valve in the nozzle. Also, the valve stem obstructs the melt flow by being in the center of the melt stream, leading to weld lines. Finally, no provision is made for two or more separate resins to be injected through the two or more sprue ports, so this method cannot be used for co-injection purposes.
U.S. Pat. No. 5,645,874 shows a multiple gate noble in which each nozzle associated with a respective gate is equipped with an individual heater to allow independent thermal gating. In this arrangement a central flow passage feeds a plurality of radially extending branch passages leading to each respective gate, and as such, cannot accommodate multiple sources of resin or even sequential melt flow, and therefore cannot be used for co-injection purposes.
U.S. Pat. No. 4,702,686 shows a nozzle wherein a tapered plate divides a central flow channel into two partial channels prior to the nozzle tip and gate. This nozzle cannot accommodate the separate, different, resin sources require for coinjection purposes.